[pdftex] calculating in LaTeX

[Stueker, Dirk]

I want to define a new command like this:
\newcommand{\mycommand}[2]{\begin{figure} \includegraphics[height=#2,
width=2*#2]{#1} \end{figure}}

Every graphic should be twice as wide as high. How can I achieve this? The
proposed solution does
not work, because 2*#2 is not evaluated.

Is there any way to evaluate expressions? Another example could be to write
sqrt(2) instead of 1.141...

Thanks 
Dirk